This invention relates to devices for protection from damage by lightning of electrical and electronic equipment powered by conventional electrical power or attached to other circuits such as telephone and cable television circuits.
Lightning conceivably may have provided humankind with a first source of fire, but lightning has otherwise been a destructive force throughout human history. Strategies and apparatus for reducing the likelihood of damage by lightning are fairly old, including, for instance, lightning rods that have been in use for approximately 200 years. While the use of such rods and other precautionary steps and safety devices reduce the damage and injury that would otherwise result from lightning today, it remains an enormously dangerous natural phenomenon that claims hundreds of lives worldwide every year and destroys a substantial amount of property. Somewhat paradoxically, advances in other areas of technology have increased rather than diminished the damage caused by lightning. This is because relatively low voltage and current levels can damage integrated circuits and other modern electronic components, with the result that many electronic devices are more susceptible to lightning damage today than ever before, and many devices to which microprocessor technology has been added are more susceptible to lightning damage as a result of such improvements. Additionally, lightning is capable of inducing substantial currents not only in electrical circuits directly struck by it but also in circuits located within the magnetic field induced by a near by lightning strike, giving each strike enormous destructive potential. See e.g., Richard T. Hasbrouck, "Lightning--Understanding It and Protecting Systems from Its Effects," ISA paper no. 90-166, and J. Anderson Plumer, "We Need Better Lightning Protection," Fire Journal (Jan. 1987) at 41-45, 73, each of which articles are incorporated herein by reference.
Microelectronic components and circuitry are not only vulnerable to lightning but relatively ineffective as switches for use in apparatus for implementing preventive measures. This is because electrical "disconnection" achieved utilizing such circuitry nevertheless results in very close proximity of electrical components, and the voltages associated with both currents in electrical circuits induced by near by lightning strikes and direct strikes themselves can easily arc across substantial distances, frequently far exceeding the size of an entire microelectronic circuit or even the device using it. As a result, merely turning an appliance, electronic device or computer off will not protect it from lightning as long as it remains "plugged into" or otherwise connected to the power circuit. The only substantially effective protection is to unplug such devices or otherwise introduce substantial electrical insulation between the device and the power supply circuit.
The typical conductivity of indoor air may require on the order of 105 kilovolts to spark a distance of five centimeters. However, voltage surges on electrical circuits struck by or in the vicinity of lightning are often higher. Indeed, according to Hasbrouck, "Lightning--Understanding It and Protecting Systems from Its Effects," p. 611, a strike may have a peak voltage of 1.4 million volts. Thus, insulation between electrical equipment and an electrical circuit such as a power circuit must withstand a substantial voltage spike to avoid damage from a lightning-induced surge, and the insulation provided by air across a small separation between electrical contacts or components frequently is inadequate.
Numerous devices intended to deal with the risks associated with lightning or the like have previously been developed or proposed. For instance, U.S. Pat. No. 4,276,576 discloses a device that controls a relay to disconnect electrical equipment from an electric power source, controlled by logic circuitry that detects and responds to the electric field produced by charged clouds, the electric or magnetic field produced by a lightning discharge, or interruption of the electric power source.
U.S. Pat. No. 4,095,262 discloses a mechanical switch for disconnecting power and antennas connected to a television set.
U.S. Pat. No. 3,891,919 discloses a lightning detector that utilizes electronic circuitry to activate a signal device such as a light or an alarm in respond to simultaneous receipt of a signal (such as one produced by a lightning strike) at two different radio frequencies.
U.S. Pat. No. 1,859,125 discloses a device that can, in some configurations, disconnect a machine to be protected from the circuit it is in, in response to a spark or arc that generates, high frequency electromagnetic waves.
While the problem addressed by the present invention is widely and has long been recognized, and the above described and other inventions have sought to solve that problem, there remains a need for an inexpensive, highly effective device for the protection of electrically operated apparatus from damage by lightning that can be easily used, particularly in residential applications, without modification of existing electrical devices or electrical power circuits and which, in other embodiments, may be easily incorporated into existing devices.